Project Summary Glioblastoma (GBM), the grade IV glioma, is the most common and most aggressive primary brain tumor in humans. GBM is highly resistant to standard cytotoxic and molecular therapies that target tumor cells. Immunotherapies have been exploited recently, holding great promise for GBM treatment. However, immunotherapy of solid tumors remains a big challenge, largely due to insufficient infiltration and inactivation of tumor-associated T cells mediated through immunosuppressive microenvironment. The development of new therapies that target tumor microenvironment to inhibit GBM progression, to overcome tumor resistance to cytotoxic therapies, as well as to strengthen immunotherapy is urgently needed. Here, we identify a dual role of interleukin (IL)-6 in tumor immunosuppression and vascular abnormality in GBM, suggesting IL-6 blockade as a promising sole or combined therapeutic strategy for strengthening immunotherapy and chemotherapy, by activating T cells in tumor microenvironment and enhancing the delivery of drugs or T cells to the tumors. Our preliminary studies reveal that tumor-promoting M2-type macrophages are spatially proximate to ECs in human GBM, permissive for angiocrine-induced macrophage polarization. Furthermore, we revealed that IL-6 and macrophage colony stimulating factor-1 (CSF-1) induced robust arginase 1 expression in macrophages, leading to macrophage M2 polarization. We showed that IL-6 and CSF-1 induced macrophage M2 polarization through peroxisome proliferator-activated receptor (PPAR)-?-dependent transcriptional activation of hypoxia-inducible factor (HIF)-2?. Finally, utilizing a genetic orthotopic GBM model and a newly developed endothelial-specific IL-6 knockout mice, we showed that IL-6 deletion in ECs improved survival in the GBM-bearing mice, and interestingly, increased activation of tumor-associated T cells, and inhibited aberrant angiogenesis. Based on these results, we hypothesize that IL-6 is critical for alternative macrophage activation-mediated immunosuppression and aberrant tumor vascularization in GBM. We will pursue the following Aims: 1) To test experiment therapy that combines IL-6 neutralization with immune checkpoint inhibition or chemotherapy to treat GBM in preclinical mouse models; 2) To determine the in vivo role of endothelial IL-6 in macrophage M2 polarization and vascular abnormality; and 3) To define the mechanisms by which IL-6 induces macrophage M2 polarization and vascular abnormality with a focus on the regulation by HIF-2? and PPAR-?. Successful completion of this project may provide new insights into tumor immunity and lead to development of a new IL-6-targeted therapy for improving cancer immunotherapy and chemotherapy.